1. Field of the Invention
The present invention relates to measuring and control of fluid flow, particularly at low fluid flow rates. More particularly, the present invention permits such measurement and control without introducing measuring sensors or devices into the path of the fluid flow.
2. Description of the Related Art
Measurement of the flow or flow rate of a fluid in a conduit, particularly, at very low fluid flow rates, has been a problem if attempted using conventional flow sensors. At very low flow many fluid sensors do not operate properly. For example, velocity flow meters such as turbine wheel flow sensors cease to operate due to there being insufficient energy in the fluid to rotate the wheel. Differential pressure flow sensors can at times operate at low flows, but the smaller flow orifices required for low flows have been prone to obstruction if there were suspended particles in the fluid. Also, pressure drops across the orifices could be significant.
Most thermal flow sensors have the temperature sensing mechanism as a resistance bridge circuit or as a part of the entire temperature variant area whereby a change in flow has the equal and opposite effect on the two halves of the sensor. This can have a limiting effect on the range of the sensor and ambient temperature changes can affect the accuracy.
Specialized flow sensing techniques have been attempted in certain cases. For example, U.S. Pat. No. 5,035,138 used a resistive material formed of a special alloy as a tube or conduit through which a gas flowed. The special alloy was selected because of a high electrical resistivity and a high temperature coefficient of resistance. A voltage differential was applied to the resistive alloy conduit at defined positions. The resistive material of the conduit was used to heat the fluid flowing in it according to the voltage differential applied. The resistive alloy material tube acted both to heat the fluid and as an indicator of flow conditions. Spaced portions of the tube wall served as temperature sensitive resistors which developed a voltage differential as flow rates through the tube varied. The flow sensor of this patent required that the fluid tube or conduit be formed of a special resistive alloy having a high change in resistance value as a function of temperature change.